1. Field of the Invention
The present invention relates to a recording apparatus which prints out a dot image or dot pattern, such as an LED printer or LCS printer. More particularly, this invention relates to a recording apparatus which reproduces a high-density dot image corresponding to a predetermined scanning direction to thereby smoothly express step portions of an image which are formed at oblique line portions.
2. Description of the Related Art
An LED (Light Emitting Diode) printer and LCS (Liquid Crystal Shutter) printer, for example, are known as recording apparatuses which print an image on a recording sheet based on print data that is output from a host computer (host apparatus). FIG. 23A illustrates the basic structure of an LED printer 1 connected to, for example, a host computer 2. Referring to this diagram, the LED printer 1 comprises a printer controller 1a, an engine controller 1b and a PP (Page Printer) engine 1c. The printer controller 1a includes a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), a character generator and a frame memory. The printer controller 1a receives print data sent via, for example, a centronics interface from the host computer 2.
The printer controller 1a analyzes the print data from the host computer 2. If the print data is a character code, the printer controller 1a converts it into pattern data using the character generator. If the print data is one of various commands, the printer controller 1a executes an associated process. As the above processing is repeated, one page of video signals (pattern data for one page printing) is stored in the frame memory. The engine controller 1b outputs the video signals, stored in the frame memory, to a printing head which constitutes a part of the PP engine 1c to print data on a recording sheet by means of a Printing mechanism. The PP engine 1c means the whole image forming mechanism in the LED printer 1 including the printing head.
FIG. 23B presents a detailed illustration of the internal structure of the LED printer 1, and is a diagram for explaining a printing process. FIGS. 24A to 24H show time charts for the printing process. Referring to FIGS. 23B and 24A to 24H, when a horizontal sync signal HSYN shown in FIG. 24A is output to the printer controller 1a from the engine controller 1b, the printer controller 1a outputs a write valid signal VALID shown in FIG. 24D to the engine controller 1b to output one line of data of the video signals stored in the frame memory. Thereafter, the printer controller 1a outputs the video signals VIDEO, shown in FIG. 24B, to the engine controller 1b in synchronism with a clock signal VCLK shown in FIG. 24C.
More specifically, if the topmost video signals in the frame memory are for an A line, first, the video signals for the A line are output to the engine controller 1b in synchronism with the clock signal VCLK as shown in FIGS. 24B and 24C. Then, the video signals output as a signal DATA to the LED head 1c in synchronization with a clock signal DCLK as shown in FIG. 24E. After temporarily latched in response to a latch signal LAT shown in FIG. 24F, the video signals are exposed on the photosensitive surface of a photosensitive drum in response to a strobe signal STR as shown in FIGS. 24G and 24H.
Thereafter, printing will be done on the recording sheet according to the above-described processes for a B line, a C line and so forth. The photosensitive drum which is moving in the sub-scanning direction is exposed for, for example, a quarter of a write period TW (period of the horizontal sync signal HSYN) by LED elements arranged in the main scanning direction, thereby forming a dot-based electrostatic latent image on the precharged photosensitive surface. The quarter of the write period TW corresponds to a 1/4 dot. Therefore, during the quarter of TW, the exposed area moves from an area 25A to an area 25B sifted by 1/4 DOP (Dot Of Pitch) in the sub-scanning direction, as shown in FIG. 25. (The areas 25A and 25B appear at different positions for explanation in the main-scanning direction in FIG. 25. However, the areas 25A and 25B exist same position on the photosensitive drum in the main-scanning direction.)
The accumulated amount of exposure changes in the sub-scanning direction as shown in curve 25C. A developing unit applies a proper developing bias level 25D to the photosensitive surface of the photosensitive drum. The area whose amount of exposure exceeds the developing bias voltage is developed as one dot by the developing unit. Therefore, one dot 25E having one DOP in sub-scanning direction is formed on the photosensitive surface, and the dot is transferred to a recording sheet. The developing bias level is so set that the width of the sub-scanning direction is identical to the width of the main scanning direction for one dot to be formed.
The resolution which is a factor to determine the printing quality in the sub-scanning direction of the recording apparatus is determined by the write period TW. In the conventional recording apparatus, the write period TW is the same as the exposure period of the printing head as mentioned above. To improve the resolution of the conventional recording apparatus, therefore, it is necessary to reduce the size of the individual dots. This requires that the write period TW be shortened and the amount of data of the video signals which is to be stored in the frame memory be increased. In other words, increasing the resolution needs an increase in printing density from, for example, 240 DPI to 300 DPI, 300 DPI to 400 DPI, and so forth. This results in an inevitable increase in the capacity of the frame memory. Accordingly, the cost of the recording apparatus increases.